This invention relates generally to photoelectrochemical cells and particularly to such cells having a gelled electrolyte.
Considerable work has been done in the recent past on photoelectrochemical cells. One basic type of cell has one electrode which is a photo-active semiconductor while the other consists of metal. This has been referred to as a Schottky type cell because it resembles the all solid state equivalent of the same name. In the second type both electrodes are photo-active semiconductors, the anode consisting of an n-type and the cathode of a p-type semiconductor or vice versa.
Between the electrodes a suitable electrolyte is provided. This may either be an aqueous solution or a solvent or a combination of the two. This electrolyte usually includes a competitive redox-active system whose function is to eliminate either anodic oxidation or cathodic reduction. These photoreactions tend to decompose one of the electrodes. If this decomposition is allowed to occur it will quickly reduce the cell to uselessness.
Numerous workers have reported work on such cells which may make use of single or polycrystalline semiconductors. The semiconductors themselves have involved a wide range of narrow bandgap and wide bandgap materials, the bandgap being defined as the gap between the valence or filled band and the conduction band. Major emphasis has been on narrow bandgap materials because they can best capture wavelengths within ordinary sunlight and are capable of efficiently transforming solar energy into electricity.
The technology involving such cells with liquid electrolytes has been covered in U.S. Patent to Manassen et al., U.S. Pat. No. 4,064,326. This patent in particular describes a cell utilizing cadmium sulfide, cadmium selenide, cadmium telluride and other semiconductor materials. The patent also discloses various redox couples suitable for the prevention of photoelectrode corrosion or decomposition.
Cells which use solvents as electrolytes in lieu of the conventional aqueous electrolytes are reported upon in a paper by Nakatani, Matsudaira and Tsubomura which appears in the Journal of the Electrochemical Society: Electrochemical Science and Technology, Volume 125, No. 3, pages 406 to 409, March 1978. For any selected system of semiconductors and electrolytes a particular redox system must be selected. Such a redox system or couple introduced to the electrolyte ensures that a competitive redox reaction takes place at lower energy levels than those required for the decomposition of one of the electrodes. It should also be realized that the electrolyte itself is subject to decomposition.
Reference is also made to a paper by H. Gerischer which appears in the Journal Proceedings of the Electrochemical Society of a Symposium on Electrode Materials and Processes for Energy Conversion and Storage, Volume 77-6, pages 8-29 (1977). This paper discloses at length photoelectrochemical cells utilizing aqueous electrolytes, energy reactions within the cell, as well as redox couples. In this connection reference is made, for example, to FIG. 4 of the paper relating to redox reactions. FIG. 7 shows energy correlations between band edges of various semiconductors and some redox systems in aqueous electrolytes.
The subject has been treated mathematically in a paper by A. J. Nozik, entitled "Energetics of Photoelectrolysis" which appears in Proceedings of the Electrochemical Society of a Conference on the Electrochemistry and Physics of Semiconductor Liquid-Junction Solar Cells, Volume 77-3, pages 272 to 289 (1977).